1. Field of the Invention
The present invention relates to a method of operating a plant with a graduated combustion system according to the preamble of claim 1.
2. Discussion of Background
From many publications, the person skilled in the art in the field of combustion has in the meantime become conversant with the fact that premixed combustion leads to very low pollutant emissions. In this case, the NOx and CO emissions are prominent here. Great efforts are being made on all sides in order to minimize them. The premixed combustion certainly exhibits its special features and advantages in particular when the fuels used are classed among the so-called "clean fuels", which are characterized in that they contain no nitrogen combined with the fuel and no sulfur portions or sulfur compounds. A further reduction in the pollutant emissions can be achieved if the combustion in its entirety is subdivided, for example if all the combustion air available is subdivided into partial flows, these partial flows, in accordance with their quantity, being mixed with various fuel contents. Furthermore, in such a combustion technique, it is then of importance that the leanest partial flow be used for the flame stabilization. Premixed combustion also requires a backflow and mixing zone for the flame stabilization in order to ignite the inflowing fuel/air mixture by further admixing with a hot gaseous medium, for example with an exhaust gas. Numerical calculations with extensive reaction mechanisms have shown that the intensive mixing zone forming from this admixing makes a considerable contribution to the formation of NOx. One possibility of reducing the tendency of the mixing zone to form NOx is to make the fuel/air mixture even leaner. However, this measure regularly leads to the extinction of the flame in the conventional burner constructions or fuel systems.